event: New C++ AsyncTaskManager::add() no longer uses std::function

std::function has unnecessary overhead, better to just create an AsyncTask subclass in-place storing the closure

This obsoletes FunctionAsyncTask, it will be removed in a future commit

panda/src/event/asyncTaskManager.I

@@ -34,14 +34,32 @@ get_clock() {
 
 #ifndef CPPPARSER
 /**
- * Adds a new task which calls the indicated function to the task manager.
+ * Adds a new task to the task manager which calls the indicated callable.
- * Returns the newly created FunctionAsyncTask object.
+ * This method is defined as a more convenient alternative to subclassing
+ * AsyncTask.
+ *
+ * This given callable allowed to be any object defining a call operator that
+ * accepts an AsyncTask pointer and returns a DoneStatus.
+ *
+ * Returns the newly created AsyncTask object.
  *
  * @since 1.11.0
  */
+template<class Callable>
 INLINE AsyncTask *AsyncTaskManager::
-add(const std::string &name, FunctionAsyncTask::TaskFunction function) {
+add(const std::string &name, Callable callable) {
-  AsyncTask *task = new FunctionAsyncTask(name, std::move(function));
+  class InlineTask final : public AsyncTask {
+  public:
+    InlineTask(Callable callable) : _callable(std::move(callable)) {}
+
+  private:
+    virtual DoneStatus do_task() override final {
+      return _callable(this);
+    }
+
+    Callable _callable;
+  };
+  AsyncTask *task = new InlineTask(std::move(callable));
   add(task);
   return task;
 }

panda/src/event/asyncTaskManager.h

@@ -66,7 +66,8 @@ PUBLISHED:
 
   void add(AsyncTask *task);
 #ifndef CPPPARSER
-  INLINE AsyncTask *add(const std::string &name, FunctionAsyncTask::TaskFunction function);
+  template<class Callable>
+  INLINE AsyncTask *add(const std::string &name, Callable callable);
 #endif
   bool has_task(AsyncTask *task) const;